t.m.relaymanager : module documentation

Today, internet e-mail has stopped being Peer-to-peer for many problems,
spam (unsolicited bulk mail) among them. Instead, most nodes on the
internet send all e-mail to a single computer, usually the ISP's though
sometimes other schemes, such as SMTP-after-POP, are used. This computer is
supposedly permanently up and traceable, and will do the work of figuring
out MXs and connecting to them. This kind of configuration is usually
termed "smart host", since the host we are connecting to is
"smart" (and will find MXs and connect to them) rather then just
accepting mail for a small set of domains.

The classes here are meant to facilitate support for such a
configuration for the twisted.mail SMTP server